One of the most practical and effective methods of sealing roof tops, and in fact producing roofing surfaces, is the application of hot tar which may be used with a layer of felt material. The process involves the melting of the tar which in many cases is accomplished at the roofing site. The tar is provided in solid billets which are ordinarily cylindrical and wrapped in a tough paper sheathing, which is removed and the billet is inserted into a heating kettle.
The heating kettle currently exclusively sold and used in the hot tar application trade is quite simple in concept, having a large reservior or kettle through which passes a hollow pipe. The hollow pipe is the heating element and is heated by means of hot gases from a burner which represents an integral part of the mobile kettle apparatus. If there is already tar in the kettle which has solidified, it may take several hours for this pipe to warm the tar sufficiently to melt it. Because the tar is quite a good insulator, the heating pipe is ordinarily heated to a very high temperature so that the tar melts as quickly as possible.
There is no means of circulating the tar within smaller heating kettles, and a circulating pump may be used in the larger ones. In either case, there is a tendency for the tar adjacent the super heated gas pipe to pyrolize or crack, which involves, the essence, the producing of small molecules from large molecules with the result that the smaller, more volatile molecules are released into the atmosphere as a pollutant. An excellent study of this phenomenon is detailed in a publication of The Asphault Institute entitled EVALUATION OF EMISSIONS FROM ASPHAULT ROOFING KETTLES WITH RESPECT TO AIR POLLUTION by J. F. Thomas, Ph.D., of the University of California at Berkeley (April 1975). By raising the temperature only to the required working temperature, and not super heating the already melted liquid, it was found that the bulk of the fumes and solids otherwise emitted are contained in the kettle.
Although the pollution problem as well as the associated problem of coking (i.e., deposition of hard carbon deposits), of the tar on the heating pipe, represents the principal drawback of conventional tar kettles, another problem lies in the practical inability of the apparatus to be used in making small quantities of hot tar. Because approximately 3/4 of the kettle must be full in order to cover the heater pipe, and because the heating pipe must be covered to avoid fires (flashing), only large quantities of the tar may be heated at a time.